Massive garrissons are usually pointless and costly. The only garrisson size that matters is the 100 troop garrisson. Anything above this is only useful for holding down a recently conquered disloyal province to stop it revolting while it adjusts to your rule. Once you've occupied for a few years the population will adjust (and the religion if necessary). This isn't due to the actual size of the garrisson you had there, it's due to your actual occupation of the province. More troops does not increase loyalty any faster, you have to keep them loyal with a large garrissons presence as well as spies and any loyalty improving buildings.